How does the Bogen 577 quick-release attach to a camera mount, such as a shoulder mount? Does it attach to the standard screw-and-pin connectors? I've seen many references to the quick-release plates but no info on how the bottom portion is actually attached to the mount.

It has the same 1/4X20 mounting hole on the bottom of the mechanisim as any other mounting plate. Then just use the QR plate that comes with it OR the plate from a 501 or 503 head (they are the same) and you're in business.

I used the 577 on a shoulder brace for my 150's (until I stopped using it) and on my 682 monopod until I put a 501 head on it.